Ohm's Law: Universal Or Not?

Ohm's law states that the electric current through a conductor between two points is directly proportional to the voltage across the two points. However, it does not always apply.

Ohm's law is not applicable to unilateral electrical elements like diodes and transistors as they allow the current to flow through in one direction only. It also does not apply to non-linear electrical elements with parameters like capacitance, resistance, etc. as the voltage and the current won't be constant with respect to time.

Additionally, Ohm's law is only applicable to metallic conductors and does not work in the case of non-metallic conductors. It also does not apply to AC circuits, which require other principles like impedance and reactance.

Some materials and devices also do not obey Ohm's law and are called non-ohmic. These include insulators, capacitors, inductors, switches, transistors, vacuum, voltage sources, current sources, dielectrics, and semiconductors.

Ohm's Law doesn't apply to AC circuits

Ohm's Law does not apply to AC circuits without modification. This is because AC circuits involve complex sources and impedances which vary with either time or frequency.

Ohm's Law states that the electric current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance, we get the three mathematical equations used to describe this relationship:

• V=IR
• I=V/R
• R=V/I

Where I is the current through the conductor, V is the voltage measured across the conductor, and R is the resistance of the conductor.

Ohm's Law is an empirical relation that accurately describes the conductivity of the vast majority of electrically conductive materials over many orders of magnitude of current. However, some materials do not obey Ohm's Law; these are called non-ohmic.

When reactive elements such as capacitors, inductors, or transmission lines are involved in a circuit to which AC or time-varying voltage or current is applied, the relationship between voltage and current becomes the solution to a differential equation, so Ohm's Law does not directly apply since that form contains only resistances having value R, not complex impedances which may contain capacitance (C) or inductance (L).

However, Ohm's Law can be modified to apply to AC circuits by using the concept of impedance, which is the complex generalization of resistance. In this approach, a voltage or current waveform takes the form Aest, where t is time, s is a complex parameter, and A is a complex scalar. In any linear time-invariant system, all of the currents and voltages can be expressed with the same s parameter as the input to the system, allowing the time-varying complex exponential term to be canceled out and the system described algebraically in terms of the complex scalars in the current and voltage waveforms.

The complex generalization of resistance is impedance, usually denoted Z; it can be shown that for an inductor, Z=sL and for a capacitor, Z=1/(sC).

So, while Ohm's Law in its basic form does not apply to AC circuits, it can be modified to apply by taking into account the concept of impedance.

It doesn't apply to diodes

Ohm's law states that the electric current through a conductor between two points is directly proportional to the voltage across the two points. The law was named after German physicist Georg Ohm, who, in a treatise published in 1827, described measurements of applied voltage and current through simple electrical circuits containing various lengths of wire.

Ohm's law applies when considering constant-value resistive elements in a lumped-element circuit model. It is a simpler model that assumes there is a linear relationship between voltage and current for resistors.

However, Ohm's law does not apply to other materials and devices, including insulators, capacitors, inductors, switches, transistors, vacuum, voltage sources, current sources, dielectrics, semiconductors, and many others. All of these devices and materials violate Ohm's law.

Diodes are non-linear devices and do not follow Ohm's law. The relationship between voltage and current in a diode is nonlinear (or non-ohmic). As seen in the figure, the current does not increase linearly with applied voltage for a diode. One can determine a value of current (I) for a given value of applied voltage (V) from the curve, but not from Ohm's law, since the value of "resistance" is not constant as a function of applied voltage.

Therefore, it doesn't apply to diodes.

It doesn't apply to transistors

Ohm's law, in its simplest form, states that the electric current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance, we arrive at the three mathematical equations used to describe this relationship:

V = IR

I = V/R

R = V/I

Where:

• V is the voltage measured across the conductor
• I is the current through the conductor
• R is the resistance of the conductor

Ohm's law is an empirical law based on experiments that show that current is approximately proportional to the electric field for most materials. However, it is not a universal law and does not always apply. Some materials do not obey Ohm's law; these are called non-ohmic materials.

Ohm's law is not applicable to unilateral electrical elements like diodes and transistors as they allow current to flow through in one direction only. The relationship between voltage and current in these components is non-linear, and the ratio of voltage to current does not remain constant.

Diodes, PN junctions, transistors, saturated magnetic materials, etc., do not have a linear relationship between voltage and current, so the simple linear form of Ohm's law cannot be used to describe these elements.

Ohm's law is only an approximation that is true for certain circuit elements and operating parameters. These are called "linear" or "Ohmic" elements. Even the most Ohmic device can be made to act non-Ohmic if enough current is passed through it.

In conclusion, Ohm's law does not apply to transistors, as they are non-linear, unilateral electrical elements with a non-constant resistance.

It doesn't apply to non-metallic conductors

Ohm's law states that the electric current through a conductor between two points is directly proportional to the voltage across the two points. However, it does not apply to non-metallic conductors, which are referred to as "non-ohmic".

Ohm's law is a linear approximation for systems where R (resistance) is not a function of voltage. It is only an approximation that is true for certain circuit elements being used in a narrow range of operational parameters. These types of circuit elements and these types of operational parameters are called "linear" or "ohmic".

Ohm's law is commonly understood as the linear version, and it is not always obeyed. Any given material will break down under a strong-enough electric field, and some materials of interest in electrical engineering are "non-ohmic" under weak fields.

Neutral gases such as air are definitely non-ohmic. There are many different modes of ionization, so the situation gets complicated quickly and is definitely non-ohmic.

Ohm's law is not applicable for unilateral electrical elements like diodes and transistors as they allow the current to flow through in one direction only. It also does not apply to non-linear electrical elements with parameters like capacitance, resistance, etc. the ratio of voltage and current won’t be constant with respect to time, making it difficult to use Ohm’s law.

Ohm's law fails to explain the behaviour of semiconductors and unilateral devices such as diodes. It may not give the desired results if the physical conditions such as temperature or pressure are not kept constant.

It doesn't apply to semiconductors

Ohm's law states that the electric current through a conductor between two points is directly proportional to the voltage across the two points. However, it does not apply to all materials and devices.

Semiconductors, such as silicon, are one such example of a material that does not obey Ohm's law and are known as non-Ohmic conductors. This is because the ratio of voltage to current does not remain constant for variations in voltage.

Ohm's law assumes that there is a linear (affine) relationship between voltage and current. However, in semiconductors, the relationship between voltage and current is nonlinear.

In other words, semiconductors do not have a constant resistance. Instead, their resistance changes as the voltage or current is increased, which means that Ohm's law cannot be applied.

Additionally, semiconductors can have ordered electronic states, which can result in nonlinear profiles that do not follow Ohm's law.

Overall, while Ohm's law is a useful principle in electrical engineering, it has its limitations and does not apply to all materials and devices, including semiconductors.

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